1. Field of the Invention
This invention relates to a power supply device suitable for use in a telephone set, and more particularly to a power supply device which may stabilize and produce a DC voltage with a superposed AC voice/FAX signal without interfering with the AC signal.
2. Description of the Prior Art
FIG. 1 shows a conventional power supply device for a telephone set. In this power supply device, a supply voltage is applied between input voltage terminals 2 and 4. There is provided between the input terminals 2 and 4 a full-wave rectifying circuit 14 consisting of a diode bridge constituted of diodes 6, 8, 10 and 12, and is further provided with a hybrid transformer 16 on the output side of the full-wave rectifying circuit 14.
The hybrid transformer 16 includes a plurality of coils 16A, 16B and 16C, and there is provided between the connection point of the coils 16A and 16B and the lower potential side line a transmission amplifier 20 which serves to amplify an input signal from the transmitter 18.
Further, there is provided between the connection point of coils 16B and 16C and the lower potential side line a balancing circuit network 28 comprising resistances 22 and 24 and a capacitor 26, and also a capacitor 30 is inserted in the lower potential side line between the balancing circuit network 28 and the transmission amplifier 20.
Further, there is connected between coil 16C and the lower potential side line a receiving circuit 38, a series circuit comprising a diode 32, receiver 34 and capacitor 36.
In such a power supply device as above, the hybrid transformer 16 acts to separate the direct current from the alternating current. However, use of hybrid transformer 16 makes for a large size of the power supply device, and even if the transmission amplifier 20 is formed of an integrated circuit (IC), the telephone set itself cannot be made sufficiently compact with the result of poor efficiency.
In another way in which no hybrid transformer is used, it is usual that a DC current is obtained by a constant-current circuit. However, in this method, a voltage to be supplied from the telephone exchange to a telephone set is 3 V as a DC component and 3 Vpp (peak to peak) as an AC signal component in the case of a long distance area, and it is instantaneously reduced to about 1.5 V as a minimum voltage, thereby causing a voltage in the IC unit to be reduced to about 1 V. As a result, under such a condition, it is practically impossible to supply current from the constant-current circuit to a regulated power supply and conduct signal processing of the telephone set by using the output from the regulated power supply. In particular, it is greatly difficult to conduct such signal processing without varying the impedance of the constant-current circuit.